Re: [PATCH 1/2] virtio_balloon: fix pages_to_free calculation

[Khazhismel Kumykov <khazhy@google.com>]

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On Mon, Nov 18, 2019 at 3:09 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
>
> So I really think we should do something like the below instead.
> Limit playing with balloon pages so we can gradually limit it to legacy.
> Testing, review would be appreciated.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 226fbb995fb0..7cee05cdf3fb 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -772,6 +772,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb,
>         return blocks_freed << VIRTIO_BALLOON_FREE_PAGE_ORDER;
>  }
>
> +static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> +                                          unsigned long pages_to_free)
> +{
> +       return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) /
> +               VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
>  static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>                                           unsigned long pages_to_free)
>  {
> @@ -782,11 +789,9 @@ static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>          * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
>          * multiple times to deflate pages till reaching pages_to_free.
>          */
> -       while (vb->num_pages && pages_to_free) {
> -               pages_freed += leak_balloon(vb, pages_to_free) /
> -                                       VIRTIO_BALLOON_PAGES_PER_PAGE;
> -               pages_to_free -= pages_freed;
> -       }
> +       while (vb->num_pages && pages_freed < pages_to_free)
> +               pages_freed += leak_balloon_pages(vb, pages_to_free);
> +
>         update_balloon_size(vb);
>
>         return pages_freed;
> @@ -799,7 +804,7 @@ static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
>         struct virtio_balloon *vb = container_of(shrinker,
>                                         struct virtio_balloon, shrinker);
>
> -       pages_to_free = sc->nr_to_scan * VIRTIO_BALLOON_PAGES_PER_PAGE;
> +       pages_to_free = sc->nr_to_scan;
>
>         if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>                 pages_freed = shrink_free_pages(vb, pages_to_free);
>

I got confused about shrink_free_pages, since, looking at the
operations the function *should* return the same units it takes in.
However, it does look like it should be operating on system pages
(VIRTIO_BALLOON_FREE_PAGE_ORDER is the order of our real page
allocations), so the multiplication on nr_to_scan is where we messed
up. OK. Sorry for the mixup :)

Your patch looks correct to me.

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